Display element and method for producing the same

ABSTRACT

Disclosed are a production process of a display device, which can prevent the oxidation of a lower electrode and can maintain luminescence efficiency, high contract, and durability, and a display element. The display element comprises a first electrode, a luminescent layer, a second electrode, and a transparent substrate. The first electrode comprises a metal layer and a corrosion-resistant charge injection accelerating layer. The corrosion-resistant charge injection accelerating layer has been formed by subjecting a surface layer in the metal layer to plasma treatment using an oxygen atom-containing gas.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a display element, particularlyan electroluminescent display element and a process for producing thesame.

[0003] 2. Background Art

[0004] An electroluminescent display element, especially anelectroluminescent element (hereinafter often referred to as “ELelement”), basically comprises a transparent substrate and, stacked onthe transparent substrate in the following order, an anode, aluminescent layer, and a cathode and is constructed so that luminescencein the EL element occurs from the anode side (substrate side).

[0005] In order to cope with demands for diversification of screendisplay devices and the like, however, EL elements, which causeluminescence from the cathode side, have been required and developed.For example, Japanese Patent Laid-Open No. 43980/2001 proposes a cathodeluminescence-type organic EL element in which an anode in its part incontact with an organic layer side contains a metal belonging to thegroup 5 or 6 of the periodic table (particularly chromium, molybdenum,tangsten, tantalum, and niobium) which can inhibit the occurrence ofdark spots (nonluminous points) and thus can prevent unevenluminescence. Japanese Patent Laid-Open No. 216976/2002 proposes anorganic EL element which can make the roughness of the surface of thelower electrode even and can suppress the occurrence of a leak currentand dark spots by virtue of the adoption of a lower electrode comprisinga metal layer (chromium) and a buffer thin-film layer formed bysputtering an oxide of the metal having a higher electrical conductivitythan an organic layer on the metal layer.

[0006] At the present time, however, there remains an earnest desire forthe development of a display element which has been improved in currentdensity and luminescence efficiency over an indium-tin oxide (ITO)electrode.

SUMMARY OF THE INVENTION

[0007] The present inventor has now found that a luminescent element,which has improved current density and luminescent efficiency and canprevent the corrosion of the first electrode, can be realized byadopting, as an electrode corresponding to the above lower electrode, afirst electrode comprising a metal layer and a corrosion-resistantcharge injection accelerating layer formed by subjecting a surface ofthe metal layer to plasma treatment. Accordingly, an object of thepresent invention is to provide this type of luminescent element.

[0008] Thus, according to one aspect of the present invention, there isprovided a display element comprising a first electrode, a luminescentlayer, a second electrode, and a transparent substrate,

[0009] said first electrode comprising a metal layer and acorrosion-resistant charge injection accelerating layer,

[0010] said corrosion-resistant charge injection accelerating layerhaving been formed by subjecting a surface layer in said metal layer toplasma treatment using an oxygen atom-containing gas.

[0011] According to another aspect of the present invention, there isprovide a process for producing a display element comprising the stepsof:

[0012] forming a metal layer on a substrate;

[0013] performing patterning on the top of the metal layer;

[0014] subjecting the surface of the metal layer to plasma treatmentusing an oxygen atom-containing atom to convert the surface of the metallayer to a corrosion-resistant charge injection accelerating layer;

[0015] forming a luminescent layer on the corrosion-resistant chargeinjection accelerating layer; and

[0016] forming a second electrode on the luminescent layer.

[0017] In the display element according to the present invention, byvirtue of the construction such that the first electrode comprises ametal layer and a corrosion-resistant charge injection acceleratinglayer formed on a surface of the metal layer by subjecting the surfaceof the metal layer to plasma treatment, the oxidation and corrosion ofthe first electrode can be prevented to improve the durability of thefirst electrode and, at the same time, the current density andluminescence efficiency of the display element can be significantlyimproved.

[0018] Further, in the production process of a display element accordingto the present invention, by virtue of the formation of acorrosion-resistant charge injection accelerating layer on the metallayer by subjecting the surface of the metal layer to plasma treatment,after this treatment, a luminescent layer can be formed without cleaningthe substrate. This can simplify the production process and can lowerthe production cost and, at the same time, can suppress shortcircuitingderived from inclusion of fine particles and the occurrence ofnonluminous points.

BRIEF DESCRIPTION OF DRAWING

[0019]FIG. 1 is a schematic cross-sectional view showing an embodimentof the EL element according to the present invention.

[0020] Reference characters in FIG. 1 will be described.

DESCRIPTION OF REFERENCE CHARACTERS IN THE DRAWING

[0021]1: substrate, 2: first electrode, 2 a: metal layer, 2 b:corrosion-resistant charge injection accelerating layer, 3: luminescentpart, 3 a: hole injection transport layer, 3 b: luminescent layer, 3 c:electron injection layer, 4: second electrode, 4 b: protective layer,and 4 a: transparent electrode layer.

DETAILED DESCRIPTION OF THE INVENTION

[0022] Display Element of Invention

[0023] The display element of the present invention and the productionprocess of the same will be described with reference to FIG. 1. FIG. 1is a schematic cross-sectional view of an embodiment of the displayelement according to the present invention. The display element shown inFIG. 1 comprises a substrate 1, a first electrode 2, a luminescent part3 including a luminescent layer 3 b, and a second electrode 4. Inanother embodiment of the present invention, the display elementcomprises an electron injection layer 3 c provided between the secondelectrode and the luminescent layer 3 b. In a further embodiment of thepresent invention, the second electrode 4 comprises a protective layer 4b and a transparent electrode layer 4 a.

[0024] In the present invention, a metal layer 2 a constituting thefirst electrode 2 is formed on the substrate 1. Next, a surface (secondelectrode 4 side) of the metal layer 2 a is subjected to plasmatreatment using an oxygen atom-containing gas to form acorrosion-resistant charge injection accelerating layer 2 b. Next, theluminescent part 3 comprising the luminescent layer 3 b and optionallyother layer(s) is formed. Finally, the second electrode 4 is formed onthe luminescent part 3 to prepare the display element according to thepresent invention.

[0025] Substrate

[0026] In the present invention, the substrate is used as a lowersurface of the first electrode and as such does not need to betransparent. Specific examples of substrates include substrates ofquartz, glass, silicon wafers, and glass with TFT (thin-film transistor)formed thereon, or polymeric substrates of polycarbonate (PC),polyethylene terephthalate (PET), polybutylene terephthalate (PBT),polyphenylene sulfide (PPS), polyimide (PI), polyamide-imide (PAI),polyether sulfone (PES), polyether imide (PEI), polyetherether ketone(PEEK) and the like. Among them, quartz, glass, silicon wafers, orpolymeric substrates of polyimide (PI), polyamide-imide (PAI), polyethersulfone (PES), polyether imide (PEI), polyetherether ketone (PEEK) andthe like are particularly preferred. Since these substrates canwithstand a temperature of 200° C. or above, high temperature treatmentcan be carried out in the production stage.

[0027] First Electrode (Anode)

[0028] a) Metal Layer

[0029] The metal constituting the metal layer is not particularlylimited so far as the metal is electrically conductive. Examples ofmetals usable herein include chromium (Cr), nickel (Ni), tungsten (W),manganese (Mn), indium (In), tin (Sn), zinc (Zn), aluminum (Al), gold(Au), silver (Ag), tantalum (Ta), platinum (Pt), palladium (Pd),molybdenum (Mo), vanadium (V), titanium (Ti), tantalum (Ta), niobium(Nb), and a combination of two or more of the above metals, an alloycomposed mainly of the above metals, and a combination thereof. Themetal is preferably selected from the group consisting of chromium,nickel, tungsten, manganese, indium, tin, and zinc.

[0030] In a preferred embodiment of the present invention, the metallayer comprises a laminate of one or more alloys and one or more metalsor alloys. The alloy particularly preferably has excellent heatresistance and corrosion resistance, and examples of such alloys includeCr-base alloys (for example, Cr—Al—Mn—Si alloy and Cr—Mn—C—Si alloy) andNi—Cr-base alloys (for example, Cr—Ni—C—Mn alloy, Cr—Ni—Mn—Si alloy,Cr—Ni—Mo—Mn alloy, Cr—Ni—Ti—Mn alloy, Cr—Ni—Ta—Mn alloy, and Cr—Ni—Cu—Calloy). Alloys comprising nickel, titanium, tantalum, and zirconiuminclude Ti—base alloys (for example, Ti—Al—Sn alloy, Ti—Mn alloy, andTi—Al—V alloy), and Zr—Ni—base alloys (Zr—Sn—Fe alloy, Zr—Sn—Fe—Cralloy, Ni—Cr—Fe—Ti alloy, Ni—Cr—Mo—Fe alloy, Ni—Cu—Fe alloy, Ni—Cr—Fealloy, and Ni—Mn—Al—Si alloy). Further, amorphous metal alloys may alsobe preferably used. Specific examples of amorphous metal alloys includemetal-semi-metal (metal: e.g., iron (Fe), cobalt (Co), nickel (Ni), andniobium (Nb), semi-metal: e.g., phosphorus (P), boron (B), and silicon(Si)) amorphous alloys and metal-metal (e.g., Fe—Zr, lanthanum (La)—Cu,uranium (U)—Co, and Ca—Al) amorphous alloys.

[0031] Specific examples of the laminate construction of the metal layerinclude electrode/alloy in which a Cr—base or Ni—Cr—base alloy has beenstacked on a lower electrode, or electrode/amorphous metal in which anamorphous metal has been stacked on a lower electrode metal,alloy/amorphous metal, amorphous metal/alloy, or a combination such asan alternately stacked structure of these laminates.

[0032] The formation of the above metal layer can reduce protrusionsderived from metal grain boundaries and can reduce the average surfaceroughness. Therefore, a display element, in which the occurrence ofshortcircuiting and leak current has been suppressed, can be realized.Preferably, the metal layer has a resistivity of not more than 1×10⁻²Ω·cm.

[0033] Methods usable for the formation of the metal layer on thesubstrate include sputtering, vacuum heat deposition, EB deposition, andion plating.

[0034] The thickness of the metal layer formed on the substrate is notparticularly limited. However, for example, in the case of a simplematrix drive panel, the thickness of the metal layer is in the range of40 to 500 nm, preferably in the range of 100 to 300 nm. When the metallayer thickness is in the above-defined range, the resistance value canbe brought to a preferred value, and, in addition, the metal layer canbe made smooth. Further, breaking at a level difference or disconnectionof the transparent electrode layer or the like can be effectivelyprevented.

[0035] b) Corrosion-resistant Charge Injection Accelerating Layer

[0036] The corrosion-resistant charge injection accelerating layer canfunction as a charge injection layer and further can function tosuppress the corrosion of the metal layer in the first electrode.Therefore, independently of the type of the metal constituting the metallayer, the corrosion-resistant charge injection accelerating layer canaccelerate the injection of charges into the luminescent layer, canimprove the durability of the first electrode, can suppress theoccurrence of dark spots, and can realize a display element having highluminescence efficiency.

[0037] The corrosion-resistant charge injection accelerating layer isformed by subjecting a surface of the metal layer formed on the surfaceof the substrate to plasma treatment (oxidation treatment) using anoxygen atom-containing gas. The expression “oxygen atom-containing gas”as used herein refers to gases composed of oxygen molecules (forexample, O₂ and O₃) and gases containing an oxygen atom(s) as aconstituent element (for example, H₂O, CO, and C0 ₂). In the presentinvention, the plasma treatment can be carried out, for example, in sucha manner that a mixed gas composed of argon (Ar) and oxygen (O₂) isbrought to a plasma gas at a partial pressure of Ar : O₂=1:1 to 100:1,the gas pressure within a film formation atmosphere is brought to about0.5 to 0.01 Pa, and the RF output is set to 50 to 1000 W.

[0038] In the plasma treatment using an oxygen atom-containing gas, thesurface of the metal layer can be oxidized in vacuo to form acorrosion-resistant charge injection accelerating layer. Further, it isestimated that, since Ar⁺collides with the surface of the metal layer,impurities present on the surface can be removed to effectively preventthe deposition of fine particles of the impurities on the surface of themetal layer. By virtue of this, according to the present invention,unlike the prior art technique, a luminescent part can be formed on thefirst electrode without cleaning the first electrode after the formationthereof.

[0039] The thickness of the corrosion-resistant charge injectionaccelerating layer can be properly determined. Specifically, the plasmatreatment can be regulated to regulate the thickness of thecorrosion-resistant charge injection accelerating layer. The plasmatreatment can be suitably regulated by regulating the treatment time,the partial pressure of Ar:O₂, and RF output. The regulation of thethickness of the corrosion-resistant charge injection accelerating layercan improve the charge transport capacity (work function) and corrosionresistance of the first electrode.

[0040] In the present invention, the metal (alloy) in the metal layerfor the formation of the corrosion-resistant charge injectionaccelerating layer may be in an oxidized state. Rather the oxidizedstate is preferred. The metal constituting the metal layer may beoxidized, for example, by natural oxidation, plasma treatment, or ozoneUV treatment.

[0041] The thickness of the corrosion-resistant charge injectionaccelerating layer is not less than 0.1 nm and not more than 500 nm.Preferably, the upper limit of the thickness is 1 nm, and the lowerlimit of the thickness is 300 nm.

[0042] c) Characteristics

[0043] The reflectance, from the first electrode, of light incidentthrough the second electrode side is not more than 70%, preferably notmore than 60%, in the visible region (in the range of 380 to 780 nm).When the light reflectance is in the above-defined range, lightreflected from the first electrode upon incidence of external light tothe first electrode can be effectively suppressed. At the same time, thefunction of the display device as a specular surface can be preventedwithout use of any circularly polarizing film. Therefore, ahigh-contract display element can be realized at a reduced productioncost.

[0044] In the present invention, preferably, the resistivity of thecorrosion-resistant charge injection accelerating layer is lower thanthe resistivity of the luminescent layer. This is because, when theresistivity of the metal layer is low, large current density andbrightness can be realized at low voltage. Consequently, a displayelement with high luminescent efficiency can be achieved by acceleratingthe charge injection (increasing the current density). Here theresistivity (specific resistance) “ρ (Ω·cm)” means the inverse number ofelectrical conductivity “σ,” that is, ρ=1/σ. The resistance R of a partof length L in a homogeneous lead having an even cross-sectional area Sis R=(L/S)ρ. The electrical conductivity refers to a constant a in arelationship between the current density i and the electric field E inthe conductor, that is, i=σE.

[0045] Organic Layer

[0046] The organic layer in the present invention comprises aluminescent layer as an indispensable component. The organic layer mayhave a multilayer structure of the luminescent layer and optionallayer(s) which will be described later.

[0047] 1) Luminescent Layer

[0048] Materials usable for constituting the luminescent layer includeinorganic luminescent materials and organic luminescent materials. Forexample, dye luminescent materials, metal complex luminescent materialsand polymeric luminescent materials.

[0049] Specific examples of dye materials include cyclopentaminederivatives, tetraphenylbutadiene derivatives, triphenylaminederivatives, oxadiazole derivatives, pyrazoloquinoline derivatives,distyrylbenzene derivatives, distyrylarylene derivatives, silolederivatives, thiophene ring compounds, pyridine ring compounds, perinonederivatives, perylene derivatives, oligothiophene derivatives,trifumarylamine derivatives, oxadiazole dimmers, and pyrazoline dimmers.

[0050] Specific examples of metal complex materials include quinolinolaluminum complex, benzoquinolinol beryllium complex, benzoxazole zinccomplex, benzothiazole zinc complex, azomethyl zinc complex, porphyrinzinc complex, europium complex, iridium metal complex, platinum metalcomplex, and metal complexes in which the center metal is aluminum,zinc, beryllium or the like, or a rare earth metal such as terbium (Tb),eruopium (Eu), or dysprosium (Dy) while the ligand is oxadiazole,thiadiazole, phenylpyridine, phenylbenzoimidazole, quinoline or otherstructures.

[0051] In the present invention, a quinolinol aluminum metal complex(Alq3) represented by chemical formula (I) can be utilized:

[0052] Specific examples of polymeric materials includepoly-p-phenylenevinylene derivatives, polythiophene derivatives,poly-p-phenylene derivatives, polysilane derivatives, polyacetylenederivatives, polyfluorene derivatives, polyvinylcarbazole derivatives,and polymers prepared by polymerizing the above dyes and metal complexluminescent materials.

[0053] In the present invention,poly(dioctyldivinylenefluorene-co-anthracene) represented by chemicalformula (II) can be utilized:

[0054] wherein

[0055] n is not less than 5,000 and not more than 1,000,000, preferablynot less than 10,000 and not more than 800,000.

[0056] The luminescent layer is formed in a pattern form, and, in thecase of a full-color display element, luminescent layers of a pluralityof colors are each patterned.

[0057] The thickness of the luminescent layer is not less than 1 nm andnot more than 300 nm, preferably not less than 5 nm and not more than100 nm.

[0058] 2) Optional Layer

[0059] In the present invention, the organic layer may have a multilayerstructure of the luminescent layer and additional layers such as a holeinjection layer, a hole transport layer, a hole transport injectionlayer, an electron transport layer, and an electron injection layerstacked on top of one another.

[0060] a) Hole Injection Layer

[0061] In a preferred embodiment of the present invention, the holeinjection layer is formed particularly on the first electrode side. Thehole injection layer may be formed of any material without particularlimitation so far as the material can stabilize the injection of thehole from the anode into the organic luminescent layer. Specificexamples of materials usable herein include electrically conductivepolymers such as doped polyaniline, polyphenylenevinylene,polythiophene, polypyrrole, poly-p-phenylene, and polyacetylene, ororganic materials which constitute charge transfer complexes comprisingelectron-donating compounds such as organic materials including aphenylenediamine site and electron-accepting compounds such astetracyanoquinodimethane and tetracyanoethylene.

[0062] The thickness of the hole transport layer is not less than 1 nmand not more than 300 nm, preferably not less than 100 nm and not morethan 200 nm.

[0063] b) Hole Transport Layer

[0064] In an embodiment of the present invention, the hole transportlayer is formed particularly on the first electrode side. The holetransport layer may be formed of any material without particularlimitation so far as the material can stabilize the transport of thehole from the anode into the organic luminescent layer. Specificexamples of materials usable herein includeN-(1-naphthyl)-N-phenylbenzidine (α-NPD) and triphenyldiamine (TPD). Thehole transport layer preferably blocks electrons which has been injectedfrom the cathode.

[0065] The thickness of the hole transport layer is not less than 1 nmand not more than 300 nm, preferably not less than 5 nm and not morethan 100 nm.

[0066] c) Hole Injection Transport Layer

[0067] In another embodiment of the present invention, a hole injectiontransport layer, which functions both as the hole transport layer andthe hole injection layer, may be provided between thecorrosion-resistant charge injection accelerating layer and theluminescent layer.

[0068] The hole injection transport layer may be formed of any materialwithout particular limitation so far as the material exhibits thecontemplated function. Specifically, any material, which can stablytransport the hole supplied from the anode into the luminescent layerwithout particular limitation. Specific examples thereof includeN-(1-naphthyl)-N-phenylbenzidine (α-NPD),4,4,4-tris(3-methylphenylphenylamino)triphenylamine (MTDATA), andhigh-molecular weight materials such as poly-3,4-ethylenedioxythiophene(PEDOT), polyaniline derivatives, and polyphenylenvinylene derivatives.

[0069] In the present invention, polyethylenedioxythiophene (PEDOT/PSS),which is a mixture of polyethylenedioxythiophene (PEDOT; left compound)and polystyrenesulfonic acid (PSS; right compound) represented bychemical formula (III) can be utilized:

[0070] wherein

[0071] n in the left compound is not less than 5,000 and not more than1,000,000, preferably not less than 10,000 and not more than 800,000;and

[0072] n in the right compound is not less than 1,000 and not more than1,000,000, preferably not less than 3,000 and not more than 500,000. Themixing ratio between PEDOT and PSS can be freely determined.

[0073] Further, bis(N-naphthyl)-N-phenylbenzidine (α-NPD) represented bychemical formula (IV) can be utilized:

[0074] The thickness of the hole injection transport layer is notparticularly limited so far as the contemplated function can besatisfactorily exhibited. In general, however, a thickness in the rangeof 10 to 300 nm, particularly in the range of 30 to 200 nm, ispreferred.

[0075] d) Electron Transport Layer

[0076] The electron transport layer can function to transport anelectron supplied from the second electrode into the luminescent layerand is formed between the second electrode and the organic layer orbetween the protective layer and the organic layer. The electrontransport layer may be formed of any material without particularlimitation so far as the material exhibits the contemplated function.Specific examples of materials usable herein include organic materialssuch as quinolinol aluminum complex (Alq3), bathocuprone (BCP), andbathophenanthroline (Bphen). Among them, bathocuprone (BCP) andbathophenanthroline (Bphen) are preferred.

[0077] The thickness of the electron transport layer is not less than 1nm and not more than 100 nm, preferably not less than 5 nm and not morethan 50 nm.

[0078] e) Electron Injection Layer

[0079] The electron injection layer functions to transport an electronsupplied from the second electrode into the luminescent layer and isprovided between the second electrode and the organic layer, between theprotective layer and the organic layer, or between the electrontransport layer and the protective layer. When a protective layer havinga large work function is provided, the electron injection layer canrealize direct injection of an electron from the protective layer intothe luminescent layer.

[0080] The electron injection layer may be formed of any material so faras the material can exhibit the contemplated function. Specific examplesof materials usable herein include electrically conductive polymers suchas doped polyaniline, polyphenylenevinylene, polythiophene, polypyrrole,poly-p-phenylene, and polyacetylene, and organic materials whichconstitute charge transfer complexes comprising electron-donatingcompounds and electron-accepting compounds. In a preferred embodiment ofthe present invention, the material is an oxide or fluoride of an alkalimetal or an alkaline earth metal, for example, LiF, NaF, LiO₂, MgF₂,CaF₂, SrF₂, or BaF₂. These materials can facilitate electron injectionat a low voltage and can impart durability such as water resistance andheat resistance to the display element. The thickness of the electroninjection layer is preferably in the range of 0.2 to 10 nm.

[0081] In a preferred embodiment of the present invention, the electroninjection layer may be formed of a metal material per se having a workfunction of not more than 4.0 eV, specifically barium (Ba), calcium,lithium (Li), cesium (Cs), magnesium (Mg), strontium (Sr) or the like.The thickness of the electron injection layer is preferably in the rangeof 0.2 to 50 nm, particularly in the range of 0.2 to 20 nm. When thethickness of the electron injection layer is in the above-defined range,light can be taken out from the transparent electrode.

[0082] In a preferred embodiment of the present invention, the electroninjection layer is provided as an electron transport injection layerwhich functions both as the electron transport layer and the electroninjection layer. In this case, the electron transport injection layermay be formed of a mixture of an electron transport layer-constitutingmaterial with an electron injection layer-constituting material. Theelectron transport injection layer may be, for example, a layer formedby co-vapor deposition of the above-described electron transportlayer-constituting organic material, that is, Alq3 (quinolinol aluminumcomplex), BCP (bathocuprone), and Bphen (bathophenanthroline) and theabove-described electron injection layer-constituting material, that is,alkali metals or alkaline earth metals such as barium, calcium, lithium,cesium, magnesium, and strontium. In the electron transport injectionlayer formed by the co-vapor deposition, the organic material: metalmolar ratio is about 1:1 to 1:3, preferably about 1:1 to 1:2. Thethickness of the electron transport injection layer formed by theco-vapor deposition is 5 to 200 nm, preferably 10 to 80 nm. Since theelectron transport injection layer formed by the co-vapor deposition hashigh electron mobility and higher light transparency than the elementarymetal, the above thickness can be realized.

[0083]

[0084] Method for Organic Layer Formation

[0085] Methods usable for the formation of (each) layer constituting theorganic layer include a method wherein each layer is formed in a patternform from the material for constituting the layer, for example, by vapordeposition, printing, or ink jet recording, and a method wherein eachlayer is formed by coating a coating liquid containing the material forconstituting the layer, for example, by a coating method such as spincoating, casting, dipping, bar coating, blade coating, roll coating,gravure coating, flexo printing, spray coating, or self-organization(alternate adsorption or self-organization monolayer film). In general,vapor deposition is utilized for low-molecular weight materials, andother methods, particularly coating methods, are utilized forhigh-molecular weight materials.

[0086] Second Electrode (Cathode)

[0087] The second electrode comprises a (transparent) electrode layerand optionally a protective layer.

[0088] a) (Transparent) Electrode Layer

[0089] The (transparent) electrode layer may be formed of any materialwithout particular limitation so far as the material is electricallyconductive. The electrode layer, however, is preferably formed of atransparent material. Examples of such materials include electricallyconductive inorganic materials. Specific examples of such materialsinclude In—Zn—O (IZO), In—Sn—O (ITO), ZnO—Al, Zn—Sn—O, In—O, Sn—O, Zn—O,Cd—O, Cd—In—O, Cd—Sn—O, Mg—In—O, and Ca—Ga—O materials, or TiO₂, TiN,ZrN, HfN, LaB₆ and the like. Preferably, indium-containing inorganicoxides (more preferably ITO or IZO) and TiN may be mentioned. ITO andIZO have high electrical conductivity and light transmittance and lowresistivity. Therefore, the light take-out efficiency can be improved,and, at the same time, the drive voltage of the EL element can belowered. TiN has high electrical conductivity and light transmittanceand low resistivity. Therefore, the light take-out efficiency can beimproved, and, at the same time, in the step of film formation bysputtering, there is no need to introduce oxygen, and, thus, theoxidation of the organic layer and the electron injection layer can beeffectively prevented.

[0090] The light transmittance of the transparent electrode layer is notless than 50%, preferably not less than 80%, in the visible region of380 to 780 nm. When the light transmittance is in the above-definedrange, light can be efficiently taken out from the transparent electrodelayer side.

[0091] The thickness of the transparent electrode layer is preferably inthe range of 10 to 500 nm, particularly in the range of 50 to 300 nm.When the transparent electrode layer thickness is in the above-definedrange, the function and light transmittance as the second electrode aresatisfactory and durability can be imparted to the second electrode.

[0092] b) Protective Layer

[0093] In a preferred embodiment of the present invention, the secondelectrode comprises a (transparent) electrode layer and a protectivelayer. This protective layer mainly functions to protect an organiclayer (especially a luminescent layer, an electron transport layer, oran electron injection layer) and optionally functions as an electrontransport layer which transports an electron from the transparentelectrode layer.

[0094] In forming the transparent electrode layer by sputtering or thelike, the protective layer functions to protect the organic layer,especially the luminescent layer or the electron injection layer, fromhigh-energy ion bombardment.

[0095] The protective layer may be formed of any material withoutparticular limitation so far as the material has the above function.Preferably, however, the protective layer is formed of a transparentmaterial. Specific examples of preferred materials include those havinga resistivity of not more than 1×10⁻²Ω·cm, for example, aluminum (Al),silver (Ag), gold (Au), chromium (Cr), or an alloy of magnesium (Mg)with silver, an alloy of magnesium with aluminum, or an chromium- ornickel-containing alloy.

[0096] Methods usable for the protective layer formation include vacuumdeposition, sputtering, and electron beam methods. The thickness of theprotective layer is in the range of 5 to 30 nm, preferably in the rangeof 8 to 25 nm. When the protective layer thickness is in theabove-defined range, light transparency can be imparted to theprotective layer.

[0097] Light-transparent materials may also be utilized. Specificexamples of such materials include TiN, ZrN, HfN, and LaB₆. When theprotective layer is formed using the above material, there is no need tointroduce oxygen. Therefore, the oxidation of the electron injectionlayer formed of an alkali metal or an alkaline earth metal can beprevented. In this case, the thickness of the protective layer may be inthe range of 10 to 500 nm, preferably in the range of 50 to 200 nm. Whenthe protective layer thickness is in the above-defined range, theluminescence efficiency and the drive voltage function are satisfactory.

[0098] Production Process of Display Element According to Invention

[0099] The production process of the display element according to thepresent invention will be briefly described.

[0100] A metal layer is formed on a substrate. The formed metal layer isthen patterned. The surface of the patterned metal layer is oxidized byplasma treatment using an oxygen atom-containing gas to form acorrosion-resistant charge injection accelerating layer on the surfaceof the metal layer. Next, an organic layer is stacked on thecorrosion-resistant charge injection accelerating layer preferably undera degree of vacuum of not more than 1×10⁻² Pa. When the organic layer isformed in a predetermined degree of vacuum, the inclusion of gasmolecules such as water molecules or oxygen molecules in the organiclayer is prevented, and a durable display element can be provided.

[0101] In a more preferred embodiment of the present invention, aninorganic acid- or organic acid-doped electrically conductive polymercoating solvent or a water-soluble coating solvent composed mainly ofthe above electrically conductive polymer is coated onto thecorrosion-resistant charge injection accelerating layer to stack theorganic layer. As compared with the vacuum film formation method, theformation of the organic layer by the coating method can shorten thefilm formation step time and can improve the material utilizationefficiency.

[0102] Specific examples of dopants as the acceptor include Lewis acidssuch as BF₃, PF₅, AsF₅, SbF₅, and SO₃, protonic acids such as HNO₃,H₂SO₄, HclO₄, HF, HCl, FSO₃H, and CF₃SO₃H, halogens such as Br₂, I₂, andCl₂, transition metal halides such as FeCl₃, MoCl₅, SnCl₄, MoF₅, andSnI₄, and organic acids such as benzenesulfonic acid, toluenesulfonicacid, camphorsulfonic acid, polystyrenesulfonic acid, polyvinylsulfonicacid, polyhydroxyethersulfonic acid, and polybutadienesulfonic acid.

[0103] Specific examples of dopants as the donor include alkali metalssuch as lithium (Li), sodium (Na), potassium (K), and calcium (Ca) andelectrically conductive polymers such as polyaniline,polyphenylenevinylene, polythiophene, polypyrrole, poly-p-phenylene, andpolyacetylene.

[0104] After the formation of the organic layer, the protective layer isformed by sputtering or the like, and the second electrode is thenformed to prepare a display element.

[0105] Use

[0106] The display element according to the present invention can beutilized as an electroluminescent display element, especially anelectroluminescent element.

[0107] Measurement Means

[0108] The “reflectance” and “transmittance” referred to in the presentspecification are values measured with a UV spectrophotometer (UV-2200A,manufactured by Shimadzu Seisakusho Ltd.) at room temperature in theair.

EXAMPLES

[0109] The following examples further illustrate the present inventionbut are not intended to limit it.

Example 1

[0110] A substrate (a transparent glass having a size of 25 mm inlength×25 mm in width×0.7 mm in thickness; alkali-free glass NA 35,manufactured by NH TECHNO GLASS CORP.) is cleaned, and a 200 nm-thickmetal layer of chromium was then formed on the substrate by magnetronsputtering (sputtering gas: argon (Ar), pressure: 0.3 Pa, DC output: 200W).

[0111] Thereafter, patterning was carried out by photolithography(resist: OFPR-800, manufactured by Tokyo Ohka Kogyo Co., Ltd.; etchingliquid: Cr-O1N, manufactured by Kanto Chemical Co., Inc.) to form apattern of 2 mm-wide line×2.

[0112] The substrate and the metal layer were ultrasonically cleaned,and the metal layer on its surface (second electrode side) was thensubjected to plasma treatment. At the outset, argon was provided as asputtering gas, and the pressure and the RF output were brought to 1.0Pa and 100 W, respectively. The oxide layer located on the naturallyoxidized surface of the metal layer was removed. Further, plasmatreatment was then carried out using argon and oxygen as the sputteringgas under conditions of gas partial pressure Ar:O₂=1:1, pressure 1.0 Pa,and RF output 100 W for one min to oxidize the surface of the metallayer, whereby a corrosion-resistant charge injection accelerating layerwas formed on the surface of the metal layer.

[0113] Polyethylenedioxythiophene (PEDOT (PSS)) represented by chemicalformula (I) was spin coated on the patterned first electrode to form an80 nm-thick layer which was then heat dried in vacuo to form a holeinjection transport layer.

[0114] An 80 nm-thick layer ofpoly(dioctyldivinylene-fluorene-co-anthracene) represented by chemicalformula (II) was formed as a luminescent layer in a glove box underlow-oxygen conditions, that is, in an oxygen concentration of 0.8 ppm,and under low-humidity conditions, that is, in a humidity of not morethan 1 ppm (dew point: −117° C.), and the formed luminescent layer washeat dried in vacuo. Thereafter, calcium was heat deposited in vacuo toform a 3 nm-thick electron injection layer under conditions of degree ofvacuum 8×10⁻⁵ Pa and film formation rate 0.2 angstrom/sec.

[0115] A 20 nm-thick protective layer was formed by vacuum deposition ofgold under conditions of degree of vacuum 8×10⁻⁵ Pa and film formationrate 0.1 angstrom/sec.

[0116] Thereafter, an IZO electrode was formed as a transparentelectrode layer by sputtering. The transparent electrode layer wasformed by means of an opposed target-type magnetron sputtering apparatusunder film forming conditions of a mixed gas, as a sputtering gas,composed of argon and oxygen (volume ratio Ar:O₂=400:1), RF output 100W, DC output 80 W, and film formation rate 2 angstroms/sec and 5.5×10⁻²Pa. The thickness of the transparent electrode layer was 150 nm.

[0117] Thus, a display element (an organic EL element) having aluminescent area of 2 mm×2 mm was prepared by forming the firstelectrode and the second electrode which crossed each other. Upon theapplication of 8 V to this element, the current density was about 210mA/cm², and the brightness was about 950 cd/m².

Example 2

[0118] An organic EL element was prepared in the same manner as inExample 1, except that a 20 nm-thick protective layer of aluminum wasformed by vacuum deposition under conditions of degree of vacuum 1×10⁻⁴Pa and film formation rate 0.5 angstrom/sec. In this element,luminescence was observed from the second electrode side. Upon theapplication of 8 V to this element, the current density was about 210mA/cm², and the brightness was about 900 cd/m².

Example 3

[0119] A metal layer formed on a substrate was ultrasonically cleaned,and the metal layer was subjected to plasma treatment. At the outset,plasma treatment was carried out using argon as the sputtering gas underconditions of pressure 1.0 Pa and RF output 100 W. The oxide layer inthe naturally oxidized surface of the metal layer was removed, andplasma treatment was then further carried out using argon and oxygen asthe sputtering gas under conditions of gas partial pressure Ar:O₂=1:1,pressure 1.0 Pa, and RF output 100 W to oxidize the surface of the metallayer, whereby a corrosion-resistant charge injection accelerating layerwas formed on the surface of the metal layer. The treatment time ofoxygen plasma treatment was changed to 20 sec to 10 min to measure thework function of the first electrode. The work function value wasmeasured in the air with a surface analyzer AC-1 manufactured by RIKENKEIKI CO., LTD. The results are shown in Table 1. TABLE 1 Sputtering gasPlasma Work composition treatment time function, eV Example 3-1 Ar/O₂ 20sec 5.65 Example 3-2 Ar/O₂  1 min 5.84 Example 3-3 Ar/O₂  2 min 5.86Example 3-4 Ar/O₂  3 min 5.86 Example 3-5 Ar/O₂  5 min 5.83 Example 3-6Ar/O₂ 10 min 5.78

Examples 4 to 9

[0120] Organic EL elements of Examples 4 to 9 were prepared in the samemanner as in Example 1, except that the treatment time of oxygen plasmatreatment was changed to 20 sec to 10 min, the electron injection layerwas not formed, and aluminum was vapor deposited to 150 nm on the 20nm-thick gold protective layer.

Example 10

[0121] In the same manner as in Example 1, oxygen plasma treatment wascarried out to oxidize the surface of the metal layer and thus to form acorrosion-resistant charge injection accelerating layer.

[0122] Next, a 50 nm-thick hole injection transport layer was formed onthe patterned first electrode by heat deposition in vacuo of(N-naphthyl)-N-phenylbenzidine represented by chemical formula (III)under conditions of degree of vacuum 5×10⁻⁵ Pa and film formation rate 2angstroms/sec.

[0123] Thereafter, a 50 nm-thick luminescent layer was formed by heatdeposition in vacuo of aluminum-quinolinol metal complex (Alq3)represented by chemical formula (I) under conditions of degree of vacuum5×10⁻⁵ Pa and film formation rate 2 angstroms/sec.

[0124] A 0.5 nm-thick electron injection layer was formed on theluminescent layer by heat deposition in vacuo of lithium fluoride (LiF)under conditions of degree of vacuum 5×10⁻⁵ Pa and film formation rate0.1 angstrom/sec.

[0125] Next, a 20 nm-thick protective layer was formed by vacuumdeposition of aluminum under conditions of degree of vacuum 1×10⁻⁴ Paand film formation rate 0.5 angstrom/sec. Thereafter, IZO was formed asa transparent electrode layer in the second electrode by sputtering. Thetransparent electrode layer was formed by means of an opposedtarget-type magnetron sputtering apparatus under film forming conditionsof a mixed gas, as a sputtering gas, composed of argon and oxygen(volume ratio Ar:O₂=400:1), RF output 100 W, DC output 80 W, and filmformation rate 2 angstroms/sec and 5.5×10⁻² Pa. The thickness of thetransparent electrode layer was 150 nm.

[0126] Thus, an organic EL element having a luminescent area of 2 mm×2mm was prepared by forming the first electrode and the second electrodewhich crossed each other.

[0127] Luminescence from the organic EL element was observed from thesecond electrode side. Upon the application of 6 V to this element, thecurrent density was 13 mA/cm², and the brightness was about 350 cd/m².

Comparative Example 1

[0128] An organic EL element was prepared in the same manner as inExample 4, except that the oxygen plasma treatment time was changed to 5min and the first electrode was changed to ITO.

Comparative Example 2

[0129] An organic EL element was prepared in the same manner as inExample 4, except that the plasma treatment was carried out for 3 minwithout introducing oxygen.

Comparative Example 3

[0130] An organic EL element was prepared in the same manner as inExample 10, except that the plasma treatment was carried out for 3 minwithout introducing oxygen.

[0131] For this organic EL element, upon the application of 6 V, thecurrent density was 11 mA/cm², and the brightness was about 310 cd/m².

Evaluation Test

[0132] A voltage of 8 V or 6 V was applied to the display elements ofExamples 1 to 11 and Comparative Examples 1 and 2. At that time, thecurrent density was measured. The results were as shown in Table 2below. TABLE 2 Current density, Sputtering Plasma mA/cm² gas treatment(Applied voltage) composition time 8 V 6 V Example 1 Ar/O₂  1 min 210Example 2 Ar/O₂  1 min 210 Example 4 Ar/O₂ 20 sec 4.1 Example 5 Ar/O₂  1min 9.8 Example 6 Ar/O₂  2 min 3.8 Example 7 Ar/O₂  3 min 1.3 Example 8Ar/O₂  5 min 1.1 Example 9 Ar/O₂ 10 min 0.7 Example 10 Ar/O₂  1 min 13Comparative Ar/O₂  5 min 2.6 Example 1 Comparative Ar  3 min 0.9 Example2 Comparative Ar  3 min 11 Example 3

1. A display element comprising a first electrode, a luminescent layer,a second electrode, and a substrate, said first electrode comprising ametal layer and a corrosion-resistant charge injection acceleratinglayer, said corrosion-resistant charge injection accelerating layerhaving been formed by subjecting a surface layer in said metal layer toplasma treatment using an oxygen atom-containing gas.
 2. The displayelement according to claim 1, wherein said metal layer is formed of ametal selected from the group consisting of chromium (Cr), nickel (Ni),tungsten (W), manganese (Mn), indium (In), tin (Sn), zinc (Zn),molybdenum (Mo), vanadium (V), titanium (Ti), tantalum (Ta), niobium(Nb), and a mixture thereof.
 3. The display element according to claim1, wherein said metal layer comprises a laminate of one or more alloysand one or more metals.
 4. The display element according to claim 1,wherein said first electrode reflects not more than 70% of light in thevisible region incident through the second electrode side.
 5. Thedisplay element according to claim 1, wherein said corrosion-resistantcharge injection accelerating layer has a lower resistivity than theluminescent layer.
 6. The display element according to any one of claims1 to 5, which is used as an electroluminescent element.
 7. A process forproducing a display element comprising the steps of: forming a metallayer on a substrate; performing patterning on the top of the metallayer; subjecting the surface of the metal layer to plasma treatmentusing an oxygen atom-containing gas to convert the surface of the metallayer to a corrosion-resistant charge injection accelerating layer;forming a luminescent layer on the corrosion-resistant charge injectionaccelerating layer; and forming a second electrode on the luminescentlayer.
 8. The process according to claim 7, wherein, after the formationof the corrosion-resistant charge injection accelerating layer, theluminescent layer is formed on the corrosion-resistant charge injectionaccelerating layer without cleaning the substrate.
 9. The processaccording to claim 7, wherein the formation of the luminescent layer onthe corrosion-resistant charge injection accelerating layer is carriedout under a degree of vacuum of not more than 1×10−2 Pa.
 10. The processaccording to claim 7, which further comprises the step of, after theformation of the corrosion-resistant charge injection acceleratinglayer, coating a liquid composition comprised of an electricallyconductive polymer doped with an inorganic acid or an organic acid or aliquid composition comprising said electrically conductive polymer ontothe corrosion-resistant charge injection accelerating layer.
 11. Theprocess according to claim 7, wherein, in forming thecorrosion-resistant charge injection accelerating layer, the plasmatreatment is regulated to regulate the thickness of thecorrosion-resistant charge injection accelerating layer.
 12. The displayelement according to any one of claims 1 to 6, which has been producedby the process according to claim 7.